When Cassini imaged Enceladus's south pole early last year, researchers noticed plumes of what appeared to be a steamlike substance spewing from the 300-mile-wide (480-kilometer-wide) moon's crust.

At first Porco's team thought the billows might be water vapor rising from subsurface ice deposits. Then the scientists realized they were seeing something unprecedented: outer-space liquid-water geysers not unlike Yellowstone's Old Faithful.

What causes these geysers to form? According to Porco and her colleagues, unknown heat sources inside Enceladus melt ice into deposits of subsurface water. Under pressure, these water pockets burst through the icy crust in fountainlike jets.

"Once the water comes out it freezes, and that produces copious amounts of ice particles," Porco said.